Myopia (short-sightedness) is a disorder of the eye in which distant objects cannot be clearly focused, but near objects can be. Images of distant objects are brought to focus in front of the retina; that is, the focusing power of the eye is too strong ‘at distance’. The condition may be corrected by the use of a negatively powered lens, which causes the distant images to focus on or nearer the fovea. Myopia can be a serious and progressive condition that leads to increasing visual impairment despite the use of corrective lenses. It is becoming increasingly common, with some countries in South-East Asia reporting that 80% of children aged 17 years suffer from the condition.
Hyperopia (long-sightedness) is a disorder where distant objects can be focused, but near objects cannot be focused. Hyperopia may be corrected by the use of positive power lenses.
It is generally agreed that the process of normal eye development—emmetropization—is regulated by a feedback mechanism, which regulates the length of the eye to maintain good focus both at distance and at near—or emmetropia. While it is also generally agreed that this feedback mechanism is somehow disturbed in eyes with refractive error, so that the eye grows too long in myopia and not long enough in hyperopia, there has not been consensus about the nature of the feedback mechanism or how the progression of myopia and hyperopia can be controlled. Both biochemical and optical (focal defect) mechanisms have been suggested.
While it is generally assumed that the feedback stimulus is somehow related to focal defects of the eye, the matter cannot be simple because, in progressive myopia, the condition may become worse—i.e., the eye continues to lengthen excessively—even though lenses that correct for distance vision are worn.
It has been proposed that an optical feedback mechanism is somehow upset by deficiencies in the accommodative effort of the eye due to excessive near work. The deficiency is considered to manifest as lag of accommodation (imprecise and insufficient accommodation) in some myopic eyes at near, resulting in defocus, which stimulates further undesirable axial elongation of the eye.
Bifocal lenses and PALs (progressive addition lenses) in spectacles have been proposed as a possible way to relieve the accommodative stress and defocus in the hope that the stimulus for elongation would be removed. U.S. Pat. No. 6,752,499 (Aller) describes prescribing commercially available bifocal contact lenses for myopic eyes that also exhibit near point esophoria to control the progression of myopia. Both concentric distance center and near center contact lenses were employed in myopic eyes with near point esofixation disparity. The zones of the concentric distance and near zones lie within the pupil.
In U.S. Pat. No. 6,045,578 (Collins et al.) it is proposed that emmetropization is regulated by the degree and direction of a spherical aberration present at the fovea. It is proposed that young myopes have higher levels of negative spherical aberration that promotes inappropriate eye growth and that the use of ophthalmic lenses to impart positive spherical aberration will counteract axial growth and thus the progression of myopia.
In international patent publication number WO 200604440A2 (Phillips et al) it is proposed that defocus at the fovea for both distance and near vision inhibits the feedback stimulus for excessive eye growth. This publication proposes the use of a bi-focal contact lens that simultaneously provides the central retina with (a) clear vision for both distance and near and (b) myopic defocus for both distance and near. Again, the visual image will be degraded if this proposal is used.
U.S. Pat. No. 7,025,460 (Smith et al) describes the results of animal trials demonstrating that it is the optical state of the peripheral retina, not the fovea, that dominates the feedback stimulus for emmetropization. Thus, Smith et al, proposes that controlling off-axis focal points of the eye relative to the central on-axis focal points through manipulation of the curvature of field of the visual image provides a method of abating, retarding or controlling the progression of myopia and hypermetropia (impaired near vision caused by insufficient eye length).
FIGS. 1 and 2 show how Smith et al describe an eye 1 that has a positive curvature of field. Since the central on-axis image point 2 is located in front of (i.e. opposite to the direction of light) the retina 3, this eye is considered myopic when measured using standard techniques such as auto-refractors, refractor-heads or trial frames, in the manner that eye-care practitioners are familiar. In this representative eye, the off-axis peripheral image points 4 for large field angles are located behind (i.e. in the direction of light) the retina 3. Thus the eye 1 of this example is relatively hypermetropic for the peripheral visual field. FIG. 2 shows a relative field curvature graph for the eye 1, which shows that the central to mid-peripheral field 5 is myopic (focus in front of retina 3) but the mid-peripheral to far-peripheral field 6 is hypermetropic (focus behind retina 3).
FIGS. 3 and 4 show how the eye 1 of FIGS. 1 and 2 is prescribed an optical device 7 for the control of the progression of myopia, as described by Smith et al. This optical device 7 is designed so that it would generate a negative relative curvature of field 8 on the eye 1. The central, on-axis image 9 is focused sharply to the fovea 11 enabling good visual acuity. The optical device 7 introduces sufficient negative relative curvature of field 8 to focus the peripheral image points 13 more anteriorly, or in front (i.e. in the direction against the direction of light in the eye) of the retina 3.
U.S. Pat. No. 7,665,842 (Ho et al) describes providing one or more vision priority zones in which peripheral defocus or another aberration is corrected, with the lens also controlling the relative curvature of field for the peripheral retina in another region outside of the vision priority zone(s).
There remains a need for methods and optical devices that address the suggested optical (focal defect) mechanisms of progression of refractive error, particularly in relation to the progression of myopia.